1. Field of the Invention
This invention relates to a speed change gear mounted on a power transmission system of vehicles such as motor cycles and automobiles, and particularly to a speed change gear provided with a stepless V-belt transmission.
2. Description of the Prior Art
The stepless V-belt transmission has its transmission ratio expressed as a ratio of an effective radius of the driven V-pulley to an effective radius of the driving V-pulley, therefore a large range of transmission ratio will be obtainable simply from preparing each V-pulley as a large-sized one. In a conventional type of speed change gear provided with the stepless V-belt transmission, since selection of a size of each V-pulley has been the sole measure for obtaining the desired range of transmission ratio, the following problems are quite unavoidable:
(1) With use of a large-sized V-pulley for obtaining a large range of transmission ratio, the circumferential speed and centrifugal force working on the V-belt become high, and when they exceed a given value, a life of the V-belt may severely deteriorate, and thus the size of V-pulley is subjected to the certain limitation.
(2) Transmission efficiency deteriorates in accordance with the increase of transmission ratio, therefore in a case where the range of transmission ratio is set large, a severe deterioration is invited on transmission efficiency in a low speed domain, and hence it is difficult to obtain satisfactory acceleration.
(3) If a large range of transmission ratio is to be attained by using a large-sized V-pulley, the wheel base between driving and driven V-pulleys should be enlarged, and movable pulley half-bodies of the driving and driven V-pulleys are arranged generally on the opposite sides; therefore the sum of axial movements of both the movable pulley half-bodies must be taken into consideration for space, and thus a casing large enough to enclose them will have to be prepared particularly therefor.
(4) Since the transmission operates in a stepless manner, the transmission ratio cannot be changed instantaneously to a desired value. Therefore a time lag is unavoidable when increasing the transmission ratio suddenly to a desired value for accelerated drive, or for the so-called kick-down operation.